1. Technical Field of the Invention
This invention relates to a thin-film multilayered electrode of a high-frequency electromagnetic field coupling type formed on a dielectric substrate, a high-frequency resonator employing the same thin-film multilayered electrode and a high-frequency transmission line employing the same thin-film multilayered electrode.
2. Description of Prior and Non-Prior Art
In recent years, there has been a trend toward downsizing of high-frequency resonators and high-frequency transmission lines in electronic components, by using materials possessing a high dielectric constant even in frequency bands as high as microwaves, sub-millimeter waves, and millimeter waves. However, there has been a problem that, if the dielectric constant is very high, downsizing is achieved but the loss of energy will increase in inverse proportion to the cube root of the bulk.
The energy loss in high-frequency resonators or high-frequency transmission lines may be classified as consisting of conductor loss due to the skin effect, and dielectric loss depending on the dielectric material. Recently, dielectric materials with low-loss characteristics, and with high dielectric constants, are being placed into practical use. In high-frequency bands, on the other hand, high-frequency currents concentrate at a conductor surface due to the skin effect so that surface resistance (or so-called skin resistance) increases as the conductor surface is approached, thus increasing the conductor loss (Joule loss). Consequently, the conductor loss, rather than the dielectric loss, has recently become the dominant factor determining the circuit unloaded Q.
Note that the skin effect is a phenomenon, peculiar to transmission of high-frequency signals, wherein high-frequency currents attenuate exponentially inside the conductor as the surface of the conductor becomes more distant. The thin region of the conductor where electric currents flow is referred to as the skin depth, which region is approximately 2.2 .mu.m at 1 GHz for, e.g. copper. Conventionally, however, the film thickness of conductors used for electrodes of high-frequency application components has been structured sufficiently thicker than the skin depth, in order to prevent radiation loss from being caused by transmission through the electrode. Meanwhile, there have also been problems of surface roughness, etc., of substrates or electrode films in the case where the electrode is formed by the metal-plating or metal-baking technique.
Making the electrode sufficiently thicker than the skin depth has been linked to the reduction of loss. However, a technique has recently been developed of film-forming electrodes precisely on a mirror-like substrate, and it has become feasible to optimize the film thickness for structuring electrodes.
In this situation, the present applicant has proposed in Japanese Patent Application No. H6-310900, etc. published Jun. 25, 1996, a thin-film multilayered electrode in which thin-film conductors and thin-film dielectrics form alternate layers. The thin-film multilayered electrode is formed on a dielectric substrate, and the skin effect is greatly suppressed when utilizing the electrode at a predetermined frequency, by setting the dielectric constant for the dielectric substrate, the dielectric constant and the film thickness for the thin-film dielectrics, and the film thickness for the thin-film conductors to predetermined values, thereby reducing the conductor loss at high frequencies. For example, where Cu thin-film conductors and SiO.sub.2 thin-film dielectrics are alternately formed over a sapphire substrate for service at frequencies of around 1 GHz, it is possible to reduce the conductor loss in the thin-film multilayered electrode by setting the film thickness of each thin-film dielectric and each thin-film conductor to values between 1 .mu.m and 2 .mu.m.
Although sapphire dielectric substrates are generally and often employed for precise formation of thin-film conductors or thin-film dielectrics as stated above, they are very expensive because they are manufactured by mirror-finish grinding from alumina single crystals. In recent times, there is further strengthening of the demand for downsizing and cost-reduction of high-frequency resonators and high-frequency transmission lines, and the possibility is being considered of forming thin-film multilayered electrodes by employing ceramic substrates, which are higher in dielectric constant than sapphire substrates and lower in cost.
It is noted that in the present specification the "ceramic substrate" referred to is generally a dielectric substrate sintered by thermal treatment of dielectric material in powder form at a predetermined temperature. The dielectric substrate has a number of pores (hereinafter referred to as the "pores" in the specification) existing in the surface thereof because of being manufactured as described above, by thermal sintering treatment of powdered dielectric material at a predetermined temperature.
Due to these pores, a problem has been that where a thin-film multilayered electrode is formed on a ceramic substrate possessing a higher dielectric constant than the sapphire substrate while using thin-film dielectrics with a relatively low dielectric constant, a short-circuit is apt to occur between the thin-film conductors formed above and below the thin-film dielectric in areas inside or around the pores in the ceramic substrate surface, preventing reduction of the conductor loss.
Another problem has been that, where a thin-film multilayered electrode is to be formed on the ceramic substrate, it takes much time and expense to form the thin-film dielectrics, due to the problems of stripping off of the thin-film dielectric and occurrence of cracks in the thin-film dielectric, which have reduced the reliability of the thin-film multilayered electrode.
Therefore, due to these problems in the formation of a thin-film multilayered electrode on a ceramic substrate possessing a higher dielectric constant than the known sapphire substrate, as described above, inexpensive and compact high-frequency resonators with high unloaded Q and high-frequency transmission lines have been unavailable.
It is an advantage of the present invention that it solves the above problems and provides a thin-film multilayered electrode which can be formed on a dielectric substrate such as a ceramic substrate, with high reliability and at low cost and further with reduced conductor loss.
The present invention also advantageously solves the above problems and provides an inexpensive, small high-frequency resonator having increased unloaded Q.
A further advantage of the present invention is to provide a small and inexpensive high-frequency transmission line which has reduced transmission loss.